1. Field of the Invention
This invention pertains to power drills for automatically drilling holes through a workpiece. More particularly, the invention pertains to devices for monitoring the operation of the drill and for automatically controlling its operation so that the drill performs in a desired manner.
2. Description of the Prior Art
There are many inventions in the prior art which in some manner automatically control the operation of a power drill. For instance, L. W. Hirsch in U.S. Pat. No. 3,051,023 describes an automatic drill having an axial thrust sensor which monitors the drilling resistance encountered by a drill bit and which retracts the drill bit to avoid breaking the bit when the drilling resistance is excessive. Hirsch, however, makes no provision for sensing when the drill has penetrated through the workpiece. Under circumstances where normal drilling resistance is encountered, the drill bit retracts only when it has advanced a preselected distance.
U.S. Pat. No. 2,857,789 by Robinson is similar to that of Hirsch in that the drill bit automatically retracts when the drilling resistance is excessive. After the drill bit has penetrated the workpiece, it advances rapidly to a preselected position and then automatically retracts. Again, no provision is made for sensing drill bit break-through and for the retraction of the bit immediately thereafter
U.S. Pat. No. 3,259,023 by R. F. Rieger et al., U.S. Pat. No. 3,584,524 by Langenbach, U.S. Pat. No. 3,107,903 by Newton, and U.S. Pat. No. 3,545,310 by Porath et al. similarly deal with devices for avoiding excessive loads on drill bits in power drills so as to reduce drill breakage, by stopping the advance of the drill bit or by automatically retracting the drill bit or by giving a warning signal. None of these patents provide for the immediate retraction of the bit following drill bit break-through.
In the inventions of the prior art, when the drill bit has penetrated through the workpiece, the bit continues to advance until reaching some preselected position. Where there are no parts of an assembly located adjacent to the far surface of the workpiece, penetration by the drill through the workpiece to a point beyond that necessary to complete the hole causes no problem and the inventions of the prior art can be used successfully in such circumstances. However, where some parts of an assembly are located in close proximity to the far side of the workpiece, some means must be included for limiting the penetration of the drill bit beyond that necessary to complete the hole. If an automatic drill is used to drill only flat panels of constant thickness, a preselected limit switch, such as that described in the prior art, can be used to avoid excessive penetration. However, if the panels being drilled are not flat or if they vary in thickness, means other than those described in the prior art must be used to avoid damaging assemblies located near the far surface of the workpiece. The invention disclosed in this specification senses the moment when the drill bit breaks through the far surface of the workpiece and causes the drill bit to retract immediately after the hole is completed, thus avoiding damage to nearby assemblies. Furthermore, the invention needs no adjustment to drill holes in surfaces of different thickness. Also, because the drill advances only as far as is needed to complete drilling of the hole, the time required to drill each hole is reduced.